798 Part V / Movement
the swing phase until the stimulus has ended (Figure
33–9A). Sensory fibers from both types of receptors are
active during stance, with the intensity of the signal
from the Golgi tendon organs being strongly related to
the load carried by the leg. Golgi tendon organs have
inhibitory actions on ankle extensor motor neurons
when the body is at rest (Chapter 32) but an excitatory
action during walking. This reversal of the sign of the
reflex is caused by inhibition of inhibitory interneuron
pathways together with a release of excitatory path-
ways during locomotion. The functional consequence
of this reflex reversal during locomotion is that the
swing phase is not initiated until the extensor muscles
are unloaded and the forces exerted by these muscles
are low, as signaled by a decrease in activity from the
Golgi tendon organs near the end of stance.
In sum, proprioceptive signals from the ankle
extensor muscles and hip flexor muscles work syner-
gistically to facilitate the stance-to-swing phase transi-
tion. In the late stance phase, when the limb is unloaded,
as inhibitory signals from Golgi tendon organs wane,
their effects on extensor rhythm generation declines,
while at the same time the activity in muscle afferents
around the hip joint is increased, facilitating activity in
flexor rhythm generation.
At least three excitatory pathways transmit sensory
information from extensor muscles to extensor motor
neurons during walking: a monosynaptic pathway from
primary muscle spindles (group Ia afferents), a disyn-
aptic pathway from primary muscle spindles and Golgi
tendon organs (group Ia and Ib afferents), and a polysyn-
aptic pathway from primary muscle spindles and Golgi
tendon organs that includes interneurons in the exten-
sor rhythm generator (Figure 33–9B). These pathways all
contribute to phase transition from stance to swing when
the ankle is unloaded and maintain extensors in stance
phase when the ankle is loaded.
In addition to regulating the transition from stance
to swing, proprioceptive information from muscle
spindles and Golgi tendon organs contributes signifi-
cantly to the generation of burst activity in extensor
motor neurons. Reducing this sensory input in cats
diminishes the level of extensor activity by more than
half; in humans, it has been estimated that up to 30% of
the activity of ankle extensor motor neurons is caused
by feedback from the extensor muscles.
Mechanoreceptors in the Skin Allow Stepping to
Adjust to Unexpected Obstacles
Mechanoreceptors in the skin, including some noci-
ceptors, have a powerful influence on the CPG for
Figure 33–8 Hip extension initiates the transition from
stance to swing phase of walking.
A.In an immobilized decerebrate cat, passive oscillating move-
ment around the hip joint initiates and entrains the fictive loco-
motor pattern in knee extensor and flexor motor neurons. The
flexor electromyogram (EMG) bursts correspond to the swing
phase and are generated when the hip is extended. (Adapted,
with permission, from Kriellaars et al. 1994.)
B.In a walking decerebrate cat, stretching of the hip flexor
muscle (iliopsoas) inhibits knee extensor EMG activity, allow-
ing knee flexor activity to begin earlier. The arrow in the knee
flexor record indicates when activity in the muscle would have
begun had the hip flexor muscle not been stretched. Activation
of sensory fibers from muscle spindles in the hip flexor muscle
is responsible for this effect. (Adapted, with permission, from
Hiebert et al. 1996.)
A 摆动髋关节
膝
屈肌
髋
伸展
髋
弯曲
膝
伸肌
1 秒
500
毫秒
摆动
髋关节
伸展
屈肌
B
伸展髋屈肌
膝伸肌
屈肌屈肌
伸展髋屈肌
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